The films are particularly suitable for outdoor applications, e.g. for greenhouses, and roofing systems. The films are also highly suitable for protective coverings, and therefore for the protection of metallic surfaces, to which the films may be hot-sealed. After even a short period in outdoor applications, films in which no UV-absorbing materials are present exhibit yellowing and impairment of mechanical properties, due to photooxidative degradation by sunlight. The films and items produced therefrom are moreover particularly suitable for applications where fire protection or flame retardancy is demanded.
Sealable, biaxially oriented polyester films are known from the prior art. There are also known sealable, biaxially oriented polyester films which have one or more UV absorbers. These films known from the prior art either have good sealing performance, or good optical properties, or acceptable processing performance.
GB-A 1 465 973 describes a coextruded, two-layer polyester film, one layer of which is composed of isophthalic acid-containing copolyesters and of terephthalic-acid-containing copolyesters and the other layer of which is composed of polyethylene terephthalate. No useful information is given concerning the sealing performance of the film. Due to lack of pigmentation, the film is not capable of production by a reliable process, i.e. is not capable of winding and has only limited further processing capability.
EP-A 0 035 835 describes a coextruded, sealable polyester film whose sealable layer comprises particles whose average particle size exceeds the thickness of the sealable layer, to improve winding and processing performance. The particulate additives form surface protrusions which inhibit undesired blocking and adhesion of the film to rollers or guides. No further detail is given concerning incorporation of antiblocking agents in the other, nonsealable, layer of the film. It is uncertain whether this layer comprises antiblocking agents. The selection of particles whose diameter is greater than the thickness of the sealable layer, with the concentrations given in the examples, impairs the sealing performance of the film. No information is given concerning the sealing temperature range of the film. The seal seam strength is measured at 140° C. and is in the range from 63 to 120 N/m (from 0.97 N/15 mm to 1.8 N/15 mm of film width).
EP-A 0 432 886 describes a coextruded multilayer polyester film which has a first surface on which a sealable layer has been arranged and which has a second surface on which an acrylate layer has been arranged. Here again, the sealable outer layer may be composed of isophthalic-acid-containing and terephthalic-acid-containing copolyesters. The coating on the reverse side gives the film improved processing performance. No information is given concerning the sealing temperature range of the film. Seal seam strength is measured at 140° C. For a sealable layer of thickness 11 μm the seal seam strength given is 761.5 N/m (11.4 N/15 mm). A disadvantage of the acrylate coating on the reverse side is that this side loses sealability with respect to the sealable outer layer. The film therefore has very restricted use.
EP-A 0 515 096 describes a coextruded, multilayer sealable polyester film which comprises an additional additive on the sealable layer. The additive may, for example, comprise inorganic particles, and is preferably applied in an aqueous layer to the film during its production. The intended result is that the film retains its good sealing properties and has good processability. Only very few particles are present in the reverse side, and these mainly pass into this layer via the regrind. This specification again gives no information concerning the sealing temperature range of the film. Seal seam strength is measured at 140° C. and is more than 200 N/m (3 N/15 mm). The seal seam strength given for a sealing layer of 3 μm thickness is 275 N/m (4.125 N/15 mm).
WO 98/06575 describes a coextruded multilayer polyester film which comprises a sealable outer layer and a nonsealable base layer. This base layer may have been built up from one or more layers, the inward-facing layer of these being in contact with the sealable layer. The other (outward-facing) layer then forms the second nonsealable outer layer. Here again, the sealable outer layer may be composed of isophthalic acid-containing and terephthalic-acid-containing copolyesters, but no antiblocking particles are present in these. The film also comprises at least one UV absorber, added to the base layer in a ratio by weight of from 0.1 to 10%. The UV absorbers used here preferably comprise triazines, e.g. Tinuvin® 1577 from Ciba. The base layer has conventional antiblocking agents. The film has good sealability, but does not have the desired processing performance and also has shortcomings in optical properties, such as gloss and haze.
DE-A 23 46 787 describes a flame-retardant polymer. Besides the polymer, its use for the production of films and fibers is also described. When this phospholane-modified polymer is used for film production the following shortcomings are apparent.                The polymer mentioned is susceptible to hydrolysis and has to be very effectively predried. When the polymer is dried using dryers of the prior art it cakes, and production of a film is therefore possible only under extremely difficult conditions.        The resultant films produced under uneconomic conditions embrittle when exposed to heat, i.e. their mechanical properties are impaired by embrittlement, so that the film is unusable. This embrittlement arises after as little as 48 hours of exposure to heat.        
It was an object of the present invention to provide a transparent, UV-resistant, flame-retardant, sealable and biaxially oriented polyester film which does not have the disadvantages of the films mentioned of the prior art and in particular has the combination of advantageous properties represented by very good sealability, cost-effective production, improved processability and improved optical properties.
It was an object of the present invention to extend the sealing temperature range of the film to low temperatures, to increase the seal seam strength on the film, and at the same time provide film handling which is better than that known from the prior art. It was also to be ensured that the processability of the film extend to high-speed processing machinery. During film production it should be possible to introduce directly arising regrind at a concentration of up to 60% by weight, based on the total weight of the film, into the extrusion process without any significant resultant adverse effect on the physical properties of the film.
Since the film is intended in particular for outdoor applications and/or critical indoor applications, it should have high UV resistance. High UV resistance means that sunlight or other UV radiation causes no, or only extremely little, damage to the films. In particular, the films are not to yellow after some years in outdoor applications, nor embrittle, nor exhibit surface cracking, nor undergo any impairment of mechanical properties. High UV resistance means that the film absorbs UV light and transmits light only when the visible region has been reached.
Flame retardancy means that in what is known as a fire protection test the transparent film complies with the conditions of DIN 4102 Part 2 and in particular the conditions of DIN 4102 Part 1, and can be allocated to construction materials classes B2 and in particular B1 for low-flammability materials.
The film is also intended to pass the UL-94 test (Vertical Burning Test for Flammability of Plastic Material) and therefore to be capable of grading in class 94 VTM-0. This means that burning of the film has ceased 10 seconds after the bunsen burner has been removed, and that after 30 seconds no smoldering is observed, and there are no burning drops.
Cost-effective production includes the capability of the raw materials or raw material components needed for production of the flame-retardant film to be dried using industrial dryers of the prior art. It is important that the raw materials do not cake and do not undergo thermal degradation. These industrial dryers of the prior art include vacuum dryers, fluidized-bed dryers, and fixed-bed dryers (tower dryers).
These dryers operate at temperatures of from 100 to 170° C., at which the flame-retardant polymers mentioned in the prior art cake, making film production impossible.
In the vacuum dryer which provides the mildest drying conditions, the raw material for the film traverses a temperature range from about 30 to 130° C. under a vacuum of 50 mbar. After this, a process known as postdrying is required in a hopper at temperatures of from 100 to 130° C., with a residence time of from 3 to 6 hours. Even here, the raw material mentioned cakes to an extreme extent.
No embrittlement or exposure to heat means that after 100 hours of heat-conditioning at 100° C. in a circulating-air oven the film has not embrittled and does not have disadvantageous mechanical properties.
Good mechanical properties include high modulus of elasticity (EMD>3200 N/mm2; ETD>3500 N/mm2) and also good values for tensile stress at break (in MD>100 N/mm2; in TD>130 N/mm2).